Homework Help: physics

Posted by lenny on Tuesday, April 24, 2012 at 4:59pm.

A hypodermic needle consists of a plunger of circular cross-section that slides inside a hollow cylindrical syringe. When the plunger is pushed, the contents of the syringe are forced through a hollow needle (also of circular cross section). If a 4.0- N force is applied to the plunger and the diameters of the plunger and the needle are 1.2 cm and 2.5 mm, respectively, what force is needed to prevent fluid flow at the needle?

A)0.17N B)0.27N C)0.43N D)0.83N E)2.7N

plzz ineed help i have no idea what im supposed to do here!

physics - bobpursley, Tuesday, April 24, 2012 at 5:26pm
The pressures are the same:

force1/area1=force2/area2

force1/diameter1^2=force2/diameter2^2

force2=force1*(diameter2/diameter1)^2

force1=4N*(.25cm/1.2cm)^2=.174N check that
physics - lenny, Saturday, April 28, 2012 at 6:15am
thanks a lot i appreciate it

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